In the conventional molding machine of this type, in order to prevent a blow-core member, which is fitted airtightly into the mouth portion of a perform in a mold, from leaking caused by floating by the pressure of air blow, a moving block fixed with the blow-core member is applied to lock for locating by a locking device. (e.g. reference patent 1: Japanese Patent Publication No. 06-218802 (Page 1, FIG. 1)).
According to the locking device in the reference patent 1, a stationary block set up on a base is locked with a moving block of the internal vertically movable blow-core member by fitting of an air-driven locking pin and locking hole. However, the problem to be solved is that locking by fitting a locking pin into a pin hole is not suitable for the conventional blow molding machine, in which the vertically movable holding plate of the blow core member is supported on pillars set up on the base as in the reference patent 1, since there is no stationary block and moving block equipped for the locking device.
Also, according to the device in the reference patent 1, because the stroke of the locking pin is long, the air blow which must be followed after the lock state is detected at the position of locking pin, and moving of the core upwardly after molding must be done after confirmation of unlock state. This results an inconvenience as locking and unlocking require unexpected time, which causes a delay of dry cycle time in the molding machine.
Moreover, in the method of locking by the locking pin and the pin hole, the holding force applied by the mean for moving the blow-core member from above to hold the blow-core member is received by the locking pin on both sides at the lower limit position of the movable block, so that the holding force given by the mean for moving the blow-core member is transmitted insufficiently to the holding plate of the blow-core member, and lack of the holding force often results a leak of blow air.
Also, during the blow molding process, as the locking pin receives the force of the blow pressure, stronger than the holding force, after the molding is operated for a long period of time, the neck portion between the pin and the piston is damaged by inclining the axis of the locking pin upwardly caused by the force of the blow pressure given repeatedly, and the blow-core member is easily lifted up for the increase of the clearance caused by the abrasion on the hole wall because of the friction on the internal wall of the pin hole.
Normally, since tight fitting between the tip portion of the blow-core member and a neck mold that holds the mouth portion of the preform is kept by fitting of the tapered surface. Even if the blow-core member moves very slight to the draw direction by floating, it causes looseness in fit and the tightness. Therefore, the air leaks from the neck mold, and this makes a drop of the blow air pressure, which becomes a cause of defects in molding. For this reason, increase of the clearance caused by the abrasion on the inside wall of the pin hole is not allowable, and the fit must be performed with a minimum clearance, so that another problem to be solved is that the locking device requires a high degree of accuracy and durability even for such a simple device.